This invention relates to an optical disk of the type including guide grooves, including the so-called "afterward write-in type" or the "DRAW (Direct Read after Write) type" optical disk.
FIG. 4 shows an example of a conventional optical disk of the type including guide grooves. In this disk, the lower side surface of a transparent circular substrate 1 (formed of PMMA, PC or the like) is provided with recesses and projections concentrically or spirally formed thereon and is wholly coated with a reflection layer 5 to form guide grooves 2 and land portions 3. Preaddress pits 4 are formed beforehand. The guide grooves 2 and the land portions 3 represent projections and recesses in the reflection layer 5 as viewed from the side from which laser beams for read-out and write-in are directed (the upper side in the figure). The substrate 1 is fixed or adhered to another substrate 7 through a spacer 6 in such a manner that an inner space is formed. The reflection layer 5 constitutes an inner side surface of this inner space.
FIG. 5 is a partially enlarged front view of a conventional guide groove-containing optical disk. The tracks are formed on the land portions 3. The portion indicated by character F corresponds to a preformat portion in which preaddress pits 4 are formed in advance to indicate an address signal Portion R corresponds to a recording portion in which pits 9 are formed afterward as recorded data (pit information) using a laser spot. The reflection layer 5 functions as a recording layer and comprises, for example, an organic dye which is sublimated or vaporized by a laser spot of prescribed power.
In a write-in operation, the writing laser beam is incident from the side of the substrate 1 and the spot thereof traces the land portion, to thereby read out an address signal using light reflected from the preaddress pits 4 on the preformat portion F, and then form pits 9 on the recording portion R to record information.
In a read-out operation, the read-out laser beam is actuated to trace the land portion 3 to read out the address signal from the preaddress pits 4 in the preformat portion and the information represented by the pits 9 recorded in the recording portion R.
A tracking servo mechanism is utilized for the trace of each laser beam along the land portion. The spot diameter of the laser beam for read-out is ordinarily within the width of the land portion and is larger than the diameter of the laser beam used for write-in, and the width of the preaddress pits.
In order to cause each laser beam to correctly trace the land portion, there is necessarily demanded an optical disk with guide grooves which increase the degree of modulation, that is, the degree of optical diffraction at the guide grooves, to enlarge the tracking signal and the track cross signal. For example, in order to enlarge the output of the tracking error signal utilizing a push-pull method, the maximum degree of modulation can be obtained when the guide grooves are designed so as to be .lambda./8 in depth, l.6.mu.in track pitch and 0.8.mu.in width, where .lambda.represents the wavelength of the light, and the track pitch corresponds to the distance between the centers of a pair of guide grooves demarcating a land portion.
In order to satisfy the above conditions in the process of manufacturing the original plate of the optical disk, in a first stage, the spot of the laser beam is converged on a photoresist layer formed to a predetermined width on the glass original plate to expose the photoresist to a light while the original plate is rotated. Next, the original plate is developed and guide grooves having radially substantially U-shaped cross sections are formed on the original plate, to thereby demarcate the land portion. The guide grooves of substantially U-shaped cross section comprise side walls of photoresist and a bottom of exposed original plate; that is, the bottom of the guide groove reaches the original plate. The term "substantially U-shaped" does not constrain the relation between the width and depth of the groove, and includes a shape in which the bottom of the groove has a roundish periphery.
The main surface on which the guide grooves and the land portions are formed is coated with a reflection layer to complete the production of the original optical disk. Ordinarily, the glass original plate is used as a matrix to make a metal stamper of Ni or the like to duplicate a large number of PMMA substrates for optical disks therefrom. The duplicated PMMA substrates are coated with reflection layers to produce optical disks with guide grooves as shown in FIG. 4. However, the conventional optical disks as described above have a disadvantage in that the spots of the laser beams for write-in and read-out encroach more frequently on the edges of the guide grooves, so that the degree of modulation of the light reflected from the preformat portion is decreased and therefore the address signal is erroneously read out.